The secondary battery is now widely used not only as a power source for portable devices such as a mobile phone, a digital camera, or a laptop computer but also as a vehicle or household power source. Among others, a lithium ion secondary battery high in energy density and light in weight is an energy storage device that has become essential for everyday life.
The secondary batteries can be largely classified into a wound type and a laminated type. The battery electrode assembly of the wound-type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet are wound a plurality of times with a separator interposed therebetween to separate the positive and negative electrodes. The battery electrode assembly of the laminated-type secondary battery has a structure in which a positive electrode sheet and a negative electrode sheet are alternately laminated with a separator interposed therebetween repeatedly to separate the positive and negative electrodes from each other. The positive electrode sheet and the negative electrode sheet have coated parts in which active material layers (including case of mixed agent of active material and binder or conductor material) are formed on a collector and uncoated parts in which no active material layer is formed because an electrode terminal is connected.
In both the wound type secondary battery and the laminated type secondary battery, the battery electrode assembly is sealed together with an electrolyte in an exterior container (exterior case). One end of a positive electrode terminal is electrically connected to the uncoated part of the positive electrode sheet while the other end is pulled out of the exterior container, and one end of a negative electrode terminal is electrically connected to the uncoated part of the negative electrode sheet while the other end extends out of the exterior container. The secondary battery tends to increase in capacity year by year, which is accompanied by an increase in heat generation when a short circuit occurs, thus increasing a danger. Therefore, battery safety measure has become increasingly important.
As the example of the safety measure, there is known a technology for forming an insulating member on a boundary portion between the coated part and the uncoated part in order to prevent a short circuit between the positive electrode and the negative electrode (Patent Document 1).